1. Field of the Invention
This invention relates to the generation of supercontinuum by launching pulses of light from a light source into an optical fiber structure.
2. Discussion of the Known Art
There are applications in the fiber optics field in which a high power, low noise, broadband light source is of particular interest. For example, efforts are now being made toward spectral slicing wherein a common light source is used to generate a multitude of wavelength division multiplexed (WDM) signals. Such an application thus has the potential for replacing many lasers with a single light source. Device characterization, e.g., dispersion measurements made on specialty fibers or the determination of transmission characteristics of gratings, may also be accomplished with such a broadband source.
Supercontinuum generation involves the launching of relatively high power light pulses into an optical fiber or microstructure, wherein the pulse light undergoes significant spectral broadening due to nonlinear interactions in the fiber. Current efforts at supercontinuum generation, typically performed using light pulses having durations on the order of picoseconds (10xe2x88x9212 sec.) in kilometer lengths of fiber, have shown degradation of coherence in the generating process, however. That is, additional noise is introduced into the system during the spectral broadening process. See M. Nakazawa, et al., Coherence Degradation in the Process of Supercontinuum Generation in an Optical Fiber, 4 Optical Fiber Technology, at 215-23 (1998).
Supercontinuum light of wavelengths spanning more than an octave have been generated in microstructured and tapered optical fibers by launching light pulses having durations on the order of femtoseconds (10xe2x88x9215 sec.) into the ends of such fibers. The extreme spectra thus produced is useful, e.g., for measuring and stabilizing pulse-to-pulse carrier envelope phase and for high precision optical frequency combs. Efforts at modeling the continuum in microstructured fibers based on a modified nonlinear Schrodinger equation have been aimed at understanding the fundamental processes involved in the spectrum generation, and show that coherence is better maintained as the launched pulses are shortened in duration from the order of picoseconds to femtoseconds.
A relatively new type of germanium doped silica fiber with low dispersion slope and a small effective area, referred to herein as highly nonlinear fiber or HNLF, has recently been developed. Although the nonlinear coefficients of HNLF are still smaller than those obtained with small core microstructured fibers, the coefficients are still several times those of standard transmission fibers due to the small effective area of HNLF. Supercontinuum generation using a HNLF and a femtosecond fiber laser has been reported. See N. Nishizawa, et al., Widely Broadband Super Continuum Generation Using Highly Nonlinear Dispersion Shifted Fibers and Femtosecond Fiber Laser, 40 Japan Journal of Applied Physics, Part 2, at pages L365 to L367 (2001). As far as is known, however, generation of a low noise, coherent, octave spanning continuum has not been achieved using an all fiber device.
According to the invention, an optical fiber suitable for generation of a supercontinuum spectrum at an output end of the fiber when pulses of light from a light source of a certain wavelength are launched into an input end of the fiber, includes a number of optical fiber sections each formed of a highly non-linear fiber (HNLF) and having a zero dispersion wavelength that is within about +/xe2x88x92200 nanometers (nm) of the light source wavelength. Each of the fiber sections has a different dispersion at the light source wavelength and an effective area (Aeff) of between 5 xcexcm2 and 15 xcexcm2 at such wavelength, a dispersion slope of between xe2x88x920.04 ps/nm2-km and 0.04 ps/nm2-km at such wavelength, and a nonlinear propagation coefficient (xcex3) of at least 5 Wxe2x88x921kmxe2x88x921. The fiber sections are operatively connected, e.g., fusion spliced in series with one another from the input end to the output end of the optical fiber.
For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims.